MEK(methyl ethyl ketone) dewaxing is the process most widely used. In MEK dewaxing, the wax bearing feed is mixed with solvent and the mixture is chilled to crystallize the wax. The chilled feed is then filtered continuously. Filtration is generally carried out in rotary drum filters. The filtration zone is at the bottom of the drum with cold wash solvent introduced at the top of the drum in the form of a spray to remove occluded filtrate.
The solvent used in the process usually contains from about 45 percent to about 75 percent MEK, with the remainder toluene. The concentration of MEK is different for each feedstock depending on the viscosity of the feed. The MEK component of the solvent induces wax precipitation while the toluene component maintains the oil in solution. MEK dewaxing is described in further detail in Hobson et al., Modern Petroleum Technology 427-429 (1975) and Hengstebeck, Petroleum Processing Principles and Applications 256-257 (1959).
Continuous filters are used in lube oil dewaxing. U.S. Pat. No. 3,791,525 to Harris et al. teaches a conventional rotary filter dewaxing apparatus.
The filtration rate generally declines as openings in the filter cloth plug up with small wax and ice particles, requiring periodic washing with hot solvent to remove the materials blocking the filter cloth. The filter must be taken off stream approximately every eight hours in order to wash with the hot solvent to restore the filter to the maximum filtration capacity.
No industrial dewaxing filters are known to be provided with continuous hot wash capability. Therefore, it is an object of the present invention to provide a method and apparatus for continuously applying a hot wash solvent to a dewaxing filter in order to increase production capacity.
The filterability of the wax is also dependent on the viscosity of the solution. Feed viscosity is different for each feedstock. A paraffin distillate has a different viscosity than a light motor oil. Therefore, it is a further object of the present invention to provide a method for changing proportions of the solvent when processing different viscosity feedstocks.